3GPP IP Multimedia Subsystem (IMS) Architecture

The IP Multimedia Subsystem (IMS) is an architectural framework defined by the wireless standards body 3rd Generation Partnership Project (3GPP) for delivering IP multimedia services to mobile users based on the UTMS network. A similar system called Multimedia Domain (MMD) was defined by the 3GPP2 (a different organization) for the CDMA2000 network, which was based on the 3GPP IMS.

For the network access, the user can connect to an IMS network in various ways using IP. Direct IMS terminals (such as mobile phones, personal digital assistants (PDAs) and computers) can register directly on an IMS network, even when they are roaming in another network or country (the visited network). The only requirement is that they can use IPv6 (also IPv4 in early IMS) and run Session Initiation Protocol (SIP) user agents. Fixed access (e.g., Digital Subscriber Line (DSL), cable modems, Ethernet), mobile access (e.g. W-CDMA, CDMA2000, GSM, GPRS) and wireless access (e.g. WLAN, WiMAX) are all supported. Other phone systems like plain old telephone service (POTS -- the old analogue telephones), H.323 and non IMS-compatible VoIP systems, are supported through gateways.

The IP Multimedia Core Network Subsystem is a collection of different functions, linked by standardized interfaces, which grouped form one IMS administrative network. The key functions of the core network are:

Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information (user profiles), performs authentication and authorization of the user, and can provide information about the user's physical location.

Call Session Control Function (CSCF) is used to process SIP signaling packets in the IMS. There are three types of CSCF: Proxy-CSCF (P-CSCF), Serving-CSCF (S-CSCF) and Interrogating-CSCF (I-CSCF).

Application servers (AS) host and execute services, and interface with the S-CSCF using SIP.

MRF (Media Resource Function) provides media related functions such as media manipulation (e.g. voice stream mixing) and playing of tones and announcements.

A BGCF (Breakout Gateway Control Function) is a SIP server that includes routing functionality based on telephone numbers.

Media Resources are those components that operate on the media plane and are under the control of IMS Core functions. Specifically, Media Server (MS) and Media gateway (MGW).